feat: add support for routing an app to a specified host:port

This is useful when there is a service not managed by Dokku but should be exposed via the Dokku routing layer. As an example, some binaries (consul, nomad, vault) expose web uis, and are traditionally run on the host directly vs in a container.

Closes #4665

plugins/builder-null/commands

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+[[ " help builder-null:help " == *" $1 "* ]] || exit "$DOKKU_NOT_IMPLEMENTED_EXIT"
+source "$PLUGIN_AVAILABLE_PATH/builder-null/help-functions"
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+
+case "$1" in
+  help | builder-null:help)
+    cmd-builder-null-help "$@"
+    ;;
+
+  *)
+    exit "$DOKKU_NOT_IMPLEMENTED_EXIT"
+    ;;
+
+esac

plugins/builder-null/help-functions

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+
+cmd-builder-null-help() {
+  declare desc="help command"
+  declare CMD="$1"
+  local plugin_name="builder-null"
+  local plugin_description="No-op builder plugin"
+
+  if [[ "$CMD" == "${plugin_name}:help" ]]; then
+    echo -e "Usage: dokku ${plugin_name}[:COMMAND]"
+    echo ''
+    echo "$plugin_description"
+    echo ''
+  elif [[ $(ps -o command= $PPID) == *"--all"* ]]; then
+    true
+  else
+    cat <<help_desc
+    $plugin_name, $plugin_description
+help_desc
+  fi
+}

plugins/builder-null/plugin.toml

@@ -0,0 +1,4 @@
+[plugin]
+description = "dokku core builder-null plugin"
+version = "0.24.10"
+[plugin.config]